LED dies typically emit light over a very wide angle, typically up to 90 degrees off its center axis, the center axis being normal to the major surface of the LED die. For some applications, such as for coupling light into a thin rectangular waveguide for backlighting a thin liquid crystal display (LCD), the LED is provided with optics to cause it to only emit light generally perpendicular to its center axis within a low angle. In such a case, the LED's major surface is parallel to the emission surface of the waveguide, and the entire light emitting portion of the side-emitting lens is within the waveguide or optically coupled to a side of the waveguide.
FIG. 1 is a side view of one such prior art side-emitting LED structure 10. The LED die 12 is mounted on a submount 14, which has top electrodes contacting bottom electrodes on the LED die 12. The submount top electrodes are typically connected by leads or vias to robust terminals on the top, side, or bottom of the submount 14 for connection to a circuit board for supplying power to the LED die 12. A phosphor layer (not shown) may be formed over the LED die 12 to convert the LED emission into a different wavelength to achieve any of a variety of colors. A reflective layer 16, such as a metal layer, is formed over the top of the LED die 12 to block light from exiting normal to the LED die's major surface and to reflect essentially all light either back into the LED die 12 or out the sides.
The side-emitting LED structure of FIG. 1 has at least the following drawbacks when using it to supply light to a thin waveguide for backlighting. The structure has an undesirable wide angle side light emission, as illustrated by some light rays exiting the side at approximately 90°. The mirror 16 reflects light back into the LED die 12, which results in multiple internal reflections before the light exists the sides, where each reflection attenuates the light. The mirror 16 is not 100% reflective. And, forming a reflective layer is a complicated step.
Providing a side-emission lens over the LED die that uses total internal reflection (TIR), instead of a reflective layer 16, to direct the light parallel to the LED die's major surface avoids the above-described drawbacks. However, such a lens increases the thickness of the LED structure to well over 1 mm. The side-emitting light cannot be efficiently coupled into a waveguide whose thickness is less than the height of the side emission.
FIG. 2 is a cross-sectional view of a prior art side-emitting lens 20 that uses TIR. Various light rays are shown being emitted from the side of the lens. Such a lens is described in U.S. Pat. No. 6,473,554, incorporated herein by reference. The lens is circular and symmetrical about its center axis. Since the LED die 12 has a width generally between 0.6-1.5 mm, and the inner angles of the lens 20 must generally reflect the light parallel to the top surface of the LED die 12, the required height of the lens 20 is dictated by physics and is necessarily greater than 1 mm. For an LED of approximately 1 mm width, the required height of the lens 20 is at least 2 mm. Increasing the diameter of the lens 20 reduces the light emission angle since more of the LED light will be reflected sideways by the top surface of the lens 20.
The lens 20 could not be used to efficiently couple light into a 1 mm thick waveguide.
There are many small consumer devices that require a small backlit display. Reducing the thickness of such displays is a goal, and one way to reduce the thickness of the display is to reduce the thickness of the backlight waveguide. It would be desirable to create a side-emitting LED structure that could efficiently couple light into a thin waveguide, such as a 1 mm waveguide.